Problem: Solve for $t$ : $t - 4 = 21$
Answer: Add $4$ to both sides: $ \begin{eqnarray} \\ t - 4 &=& 21 \\ \\ {+4} && {+4} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-4 + t} &\hphantom{=}& \hphantom{21} \\ t &=& 21 {+ 4} \\ \end{eqnarray} $ Simplify. $ t = 25$